Phase III - The primary objective of this study is to evaluate the effect of nevirapine (NVP) vs. placebo on the incidence ofperinatal HIV transmission when given late during labor and to the neonates between 48 and 72 hours of age. This study will also evaluate the safety and tolerance of NVP in pregnant HIV-infected women and their infant; determine NVP pharmacokinetic parameters in a large population of neonates and evaluate relationship of these parameters with transmission of HIV and whether they are affected by other maternal HIV medications; evaluate the relationship between maternal viral load at delivery and HIV transmission between treated and control groups and determine the effect of NVP on viral loads in the HIV-infected infants.